This invention relates to an improved method of burning coal in order to power gas turbines by providing virtually complete oxidation of the coal using unmixed combustion, but without the adverse environmental consequences associated with conventional coal-fired systems.
The types of air pollution produced by coal combustion are well known and include particulate emissions such as fine particles of ash from pulverized coal firing, the oxides of sulfur, SO2 and SO3, carbon monoxide emissions, and the emission of two of the oxides of nitrogen, NO and NO2. More recently, the problem of global warming due to Greenhouse gas emissions has been recognized, and the emissions of CO2 from power plants and other combustion systems have become a matter of serious environmental concern.
The problem of global warming due to CO2 emissions from coal and other fossil fuel combustion processes involves two basic issues. First, because of the significant amount of carbon dioxide released by conventional coal-fired systems, a need exists to modify the processes so that the CO2 leaves the process in a form that allows it to be readily disposed, i.e., producing xe2x80x9csequestration readyxe2x80x9d CO2 that can be isolated and disposed of without release into the atmosphere. Second, the need exists to find improved methods for disposing of CO2 and other waste gases that are technically feasible and economically viable. Since many proposed disposal options all involve liquid CO2 (such as pumping liquid CO2 into deep parts of the ocean), it is generally accepted that for the CO2 to be xe2x80x9csequestration ready,xe2x80x9d it need not be free of all impurities, but must not contain more than small amounts of other gas components that do not liquefy under pressure at ambient temperature.
In addition to known air pollution/CO2 emission problems, the combustion of coal has certain disadvantages relative to other fuels in powering gas turbine engines. Gas turbines are the lowest capital cost systems available for generating electrical power on a large scale. However, the thermodynamic efficiency of gas turbines normally is lower than other higher capital cost systems. As a general proposition, the thermodynamic efficiency improves by increasing the inlet temperature of the gas working fluid. Thus, recent efforts to maximize turbine efficiency have focussed on increasing the turbine inlet temperature. As a result, turbine blades and other key components of gas turbines exposed to the working fluid have been engineered to tolerate higher first stage inlet gas temperatures.
Despite the possibility for increased operating temperatures in gas turbines, a known disadvantage of coal-fired systems concerns the composition of hot gases produced by coal oxidation. The exhaust gases from coal contain fly ash that can be erosive to and corrosive to most metals used for conventional gas turbine blades. Consequently, the maximum turbine inlet temperature for coal firing typically is lower than that for a xe2x80x9ccleanxe2x80x9d fuel. Although improvements in gas turbine design and metallurgy have resulted in higher inlet gas temperatures for coal-fired systems, the maximum temperatures for clean fuels also increased. Thus, the efficiency disadvantage of coal relative to clean fuels remained. Over the years, this disadvantage has prevented the lower cost of coal from being considered as the principal gas turbine fuel in this country. Thus, if a process were developed whereby coal could be burned in a manner that produced hot gases that were not erosive or corrosive, particularly at higher inlet temperatures, coal could become a much more attractive gas turbine fuel.
With respect to global warming, coal has the further disadvantage that its CO2 emissions per BTU produced are significantly higher than those of xe2x80x9ccleanerxe2x80x9d fossil fuels. Again, if the means existed whereby coal could be burned in a manner that did not result in the emission of large amounts of CO2 and/or other pollutants, this disadvantage would disappear.
U.S. Pat. Nos. 5,339,754 and 5,509,362; and 5,827,496 (incorporated herein by reference) disclose a new method of burning solid fuels such as coal known as xe2x80x9cunmixed combustionxe2x80x9d which involves the use of a catalyst that is readily reduced when in an oxidized state and readily oxidized when in a reduced state. In essence, fuel and air are alternatively contacted with the catalyst. The fuel reduces the catalyst and is oxidized to CO2 and water vapor. The air in turn oxidizes the catalyst and becomes depleted of oxygen. Thus, combustion can be effected without the need of mixing the fuel and air prior to or during the combustion process. If means are provided whereby the CO2 and water vapor and the oxygen depleted air can be directed in different directions as they leave the combustion process, then mixing can be completely avoided.
The total volume of combustion gases produced by unmixed combustion is comparable to that produced in conventional combustion. However, the volume of the CO2+water vapor steam represents only small part of the total. Those skilled in the art recognize that the cost of removing acid gases such as SO2, HCl and HF from combustion effluents by scrubbing increases with the volume of gas being scrubbed. Thus, if unmixed combustion can be done in such a manner that the acid gases leave the combustion process in the CO2+water vapor steam, the volume of gas which must be scrubbed is greatly reduced. Likewise, the cost of scrubbing becomes significantly lower.
The ""362 patent notes that operating unmixed combustion in a manner such that the acid gases leave the combustor in the CO2+water vapor steam requires an appropriate choice of catalyst. The patent is discussed in greater detail in Paper 98F-36, presented at the October 1998 meeting of the Western States Section of the Combustion Institute. The authors of the paper (hereafter referred to as the xe2x80x9cCombustion Institute paperxe2x80x9d) include R. K. Lyon, a named inventor of the ""362 patent, and J. A. Cole. The present invention involves an improvement to the basic combustion process described in the patent and 1998 paper.
The Combustion Institute paper discloses a conceptual process for using coal to power a gas turbine and reports on a series of experiments illustrating certain specific aspects of the process. One such experiment uses a fluid bed of powdered chemically pure iron oxide (i.e., FeO/Fe2O3) operated at atmospheric pressure in which the gas used to fluidize the bed is switched from air to a mixture of 5% SO2+95% N2 and back again.
Various two-step experiments were done with this setup. In the first step, a bed fully oxidized to Fe2O3 was fluidized with the 5% SO2+95% N2 at a temperature of 857xc2x0 C. A small amount of Illinois coal was then injected into the bed while the gases coming out of the bed were continuously analyzed. In the second step, the fluidizing gas was switched to air while the gases coming out of the bed were continuously analyzed. Based on this data, the Combustion Institute paper concludes that in the presence of SO2 coal is readily oxidized and that the chief carbon-containing product of this oxidation is CO2 with little or no CO being produced.
The paper attributes the ability of the solid particles of Fe2O3 to rapidly oxidize the solid particles of coal to a catalytic action by the SO2 used in the fluidizing gas. In this catalytic process, the SO2 reacts with the coal, converting it into to CO2, CO, CS2, COS, and sulfur vapor. The CO, CS2, COS and sulfur vapor are oxidized by the Fe to CO2 and SO2. The gases exiting the bed when fluidized with air contained little or no SO2 and little or no CO and CO2. From this, the Combustion Institute paper concludes that the Fe2O3 oxidized the coal to completion during the first step, i.e., while the bed was fluidized with 5% SO2+95% N2. The oxidation converted all the sulfur in the coal to SO2 and other volatile species which exited the bed during the first step of the experiment.
Another series of two-step experiments involved fluidizing the bed with N2 and injecting thiophene (C4H4S). Like the experiments with coal, when the amount of thiophene injected was small, all of the sulfur exited the bed as SO2 and other volatile species during the first step. Increasing the amount of injected thiophene, however, changed this situation. That is, injecting thiophene in excess of a specific threshold amount caused some of the sulfur to be retained in the bed during the first step and to be released as SO2 during the second air fluidization step.
The Combustion Institute paper speculates that the observed threshold is a result of FeS being formed, i.e., after thiophene reduces some of the Fe2O3 to FeO, the injection of additional thiophene causes the formation of FeS. Once formed, the FeS remains during the first step and then oxidizes to Fe2O3 and SO2 during the air fluidization step.
Given these observations, the Combustion Institute paper proposes a conceptual design for a process to use coal to power a gas turbine. As shown in the reference""s Figure 4, the Fe2O3 catalyst is used as a fluidized powder, circulating between a first fluid bed which is fluidized with steam and a second fluid bed which is fluidized with air. Figure 4 shows the transfer lines between these fluid bed as being purged with steam. The second fluid bed is fluidized with compressed air from the compressor section of a gas turbine. Within this second bed, FeO is oxidized to Fe2O3, a strongly exothermic reaction that depletes the compressed air of oxygen and heats the air. This heated compressed air is then used to drive the expander section of the gas turbine.
The Combustion Institute paper also contemplates feeding pulverized coal to the first steam fluidized bed where it reduces the Fe2O3 to FeO while being oxidized to CO2, water vapor, and fly ash. All the volatile products of combustion are swept from the fluidized bed, with the fluidization conditions being adjusted such that the fly ash can be rapidly removed by elutriation. Figure 4 calls for the fly ash to be removed from the other combustion products with a cyclone separator after which the ash goes to disposal. After heat is recovered from the remaining combustion products, water vapor is removed by condensation and the resultant CO2 and SO2 mixture are disposed of.
The Combustion Institute paper notes that the conditions under which the coal is oxidized are such that all or virtually all of the sulfur in the coal is converted to SO2 and other volatile species rather than reacting with the FeO/Fe2O3 to form FeS or other nonvolatile sulfur containing species. The formation of FeS and similar species is undesirable since, instead of being swept out of the steam fluidized bed, they would circulate to the air fluidized bed where they oxidize to SO2.
Although instructive, the process described in the Combustion Institute paper has at least three limitations. The first relates to SO2 emissions; a second concerns the efficiency with which the coal is burned; and a third involves the efficiency with which the gas turbine produces power from the heat liberated by burning the coal.
As discussed above, the paper discloses the significance of a specific threshold amount of thiophene. If the conversion of the Fe2O3 to FeO is kept below this threshold, FeS is not formed and SO2 emissions are avoided. Although the paper discloses an amount of thiophene which is one experiment exceeded this threshold, it is silent as to how one skilled in the art would use Fe2O3 was used in this experiment. Thus, while the paper reports the existence of a threshold amount of thiophene, it is silent with respect to the extent of Fe2O3 conversion at which the threshold occurs. In addition, the paper does not discuss how changing the temperature might affect the threshold amount. Nor does it teach how the threshold is affected by changes in form of the Fe2O3, for example in xe2x80x9cred mud,xe2x80x9d the byproduct of aluminum production having a high iron content. The lack of discussion on these subjects reflects an unsolved problem in using unmixed combustion.
The Combustion Institute paper notes that the 5% SO2 corresponds to the amount of SO2 produced by oxidation of high sulfur Illinois coal with Fe2O3 at elevated pressure. For lower sulfur coals, the concentration of SO2 is lower, making the coal""s oxidation rate unacceptably low. The only solution suggested to solve this problem is very expensive. The paper recommends raising the SO2 concentration by recycling SO2, i.e., by recovering SO2 from the recovered SO2+CO2 mixture and returning it to the first fluid bed.
However, if coal is to be used to power a gas turbine in an efficient manner, obviously it is essential that the coal be burned efficiently, i.e., virtually complete combustion is necessary. This requires that the rate of coal oxidation in the first fluid bed be high. Other than this expensive solution, the Combustion Institute paper is silent with respect to means for maintaining a high oxidation rate in the first fluid bed.
Efficient coal combustion also requires that the carbon content of the fly ash be low. The experiments described in the Combustion Institute paper show that coal can be rapidly oxidized to CO2, water vapor and fly ash. However, a coal particle becomes xe2x80x9cfly ashxe2x80x9d when oxidation shrinks the particle to the point that it flies out of the fluid bed. While this implies that the fly ash would have a substantial carbon content, the paper does not identify any specific carbon content. Figure 4 shows removing fly ash from the gases leaving the first fluid bed using a cyclone separator and sending the fly ash to disposal. This, however, would mean discarding a significant fraction of the coal""s valuable heat of combustion.
It is well known that the theoretical maximum efficiency of a gas turbine increases with increasing turbine inlet temperature. Thus, if a gas turbine is to operate with an acceptably high efficiency, the inlet temperature must be correspondingly high, i.e., at temperatures approaching 1500xc2x0 C. For the process shown in Figure 4, the turbine inlet temperature would be the same or slightly less than the temperature at which the second fluid bed operates. On page 10, the Combustion Institute paper teaches that the first fluid bed should be operated at a temperature of 700-900xc2x0 C. and that the second fluid bed should be operated at xe2x80x9cnearly 1500xc2x0 C. In order to provide this temperature, the ratio of coal feed to the first bed must be sufficient so that 60% of the Fe is reduced to FeO.
These teachings represent a compromise between the conflicting need to maximize turbine efficiency and avoid SO2 emissions. The paper teaches that to improve turbine efficiency, an inlet temperature greater than 1500xc2x0 C. will be necessary and that whenever the threshold for FeS formation is less than 60% conversion of the Fe2O3, some of the sulfur in the fuel will be emitted to the atmosphere as a pollutant. The sulfur in the coal is then recovered as SO2. However, as noted above, there is virtually no market for sulfur in the form of SO2 and its storage and disposal can be expensive and difficult. Elemental sulfur, on the other hand, can be readily shipped and has a substantial market. If the sulfur cannot be sold, its storage and/or disposal are relatively easy and inexpensive.
Thus, a need still exists in the art for a new method of burning coal to power gas turbines using unmixed combustion that avoids the limitations of the prior art processes discussed above.
The present invention provides an improved method of burning coal to power gas turbines using unmixed combustion but with virtually complete oxidation of the coal. The invention also permits the use of low sulfur coals, with the sulfur content of the coal being recovered as elemental sulfur, a portion of which can be recycled to the process and the remainder recovered for commercial use or disposal.
In one exemplary embodiment, the carbon-containing fly leaving the first fluidized bed reactor is sent to a second fluidized bed reactor where the ash contacts SO2 and reacts to form gaseous products, thus producing an ash of low carbon content suitable for disposal. In another embodiment, carbon-containing fly ash from the first fluid bed is not sent to a cyclone separator (as taught by the Combustion Institute paper) but rather to a high temperature filter. While the gases leaving the first fluid bed pass through the filter, the solid particles of carbon-containing ash remain in the bed for an extended time before being removed. The gases coming out of the first fluid bed contain SO2 which react with the carbon in the fly ash. The carbon in the fly ash consists of a char and thus contains at least some hydrogen. By means specified herein, the amount of SO2 present in these gases is adjusted to provide one mole of SO2 for every mole of carbon in the fly ash and one mole of SO2 for every four moles of hydrogen. Significantly, the products of the reaction are a mixture of CO2, CO, H2S, COS, CS2, and elemental sulfur. For this particular product mixture, the amount of SO2 is a stoichiometric excess. This excess of SO2 allows the carbon in the fly ash to be almost completely gasified. The gaseous reaction mixture leaving the filter thus contains elemental sulfur in addition to CO2.
Catalysts for carrying out the preferred reaction are known in the art. The gaseous reaction mixture is passed through the catalysts and the elemental sulfur is recovered. An amount of elemental sulfur sufficient to provide one mole of SO2 for every mole of carbon in the fly ash and one mole of SO2 for every four moles of hydrogen is then recycled back to the first fluid bed. The balance of elemental sulfur is recovered for sale or disposal.
In still another embodiment of the invention, the complete removal of the carbon from the fly ash is not required when forms other than elemental sulfur are acceptable. The amount of elemental sulfur recycled to the fluid bed can be adjusted accordingly.